1. Field of Invention
The present invention relates to a quadrature oscillator, more particularly, to an an oscillator for generating four quadrature signals. The quadrature oscillator of the present invention can be advantageously applied in electronic circuit and communication system.
2. Related Art
Quadrature signals play an important role in the modulation mechanism of advance communication system. For example, the image signals will automatically disappear by mixing with quadrature signals. Application of this technique for communication circuit will lower the cost of having to use filters. However, quadrature signals are usually generated from RC phase shifters and quadrature oscillators.
Oscillator is an electronic apparatus, which can generate continuous (or discrete) sinusoidal or impulse waveforms. Oscillators of sinusoidal generator usually includes LC tanks, amplify circuit and feedback circuit, and which are widely used in circuit and communication system.
The conventional quadrature generators are formed by coupling RC phase shifters with quadrature oscillators. The RC phase shifter inputs a signal from single-phase oscillator to two RC-CR circuits, which are connected in series, for generating two continuous quadrature signals which are 90 degrees out of phase relative to each other. Poly-phase filters are formed by suitable connection of resistors and capacitors in series. We can obtain quadrature signals from poly-phase filters by inputting differential frequency.
FIG. 1(A) is the conventional quadrature signal generator formed by RC phase shifters. FIG. 1(B) is the conventional quadrature signal generator formed using poly-phase filters. The quadrature signal generators, which are formed by the RC phase shifters or the poly-phase filters, contain resistors and capacitors. However, the signal""s amplitude will be attenuated due to these passive devices (resistors and capacitors). Therefore, additional power compensation circuit will be needed. The degree of device matching can affect the central frequency, amplitude and phase-error.
The quadrature generator, which is formed from differential oscillators, is composed of active devices. For example, the feedback circuit of two differential oscillators, which are formed by BJT and CMOS, can generate four quadrature signals.
FIG. 2 is a well-known differential oscillator formed from quadrature generators. Basically, this quadrature generator, which is formed from differential oscillators, needs four inductors and four branch constant-current sources. Thus, it requires a larger area for circuit layout and involves more power dissipation.
This invention relates to a quadrature oscillator for generating four quadrature signals and differential double-frequency signals. It has the advantages of requiring less circuit area and less power dissipation, and reduces the phase-error of the output signal by coupling the two output terminals having a constant-current source.
Even though the examples of present invention as disclosed above are very descriptive, however, they are not limitative of the present invention. Any person with skills in these techniques can make certain changes and modifications, within the claims and spirit of this invention, for more applications. Therefore, the protections of this invention are according to the claims provided at the end of this disclosure.